In numerous applications, it is desirable in injection molding to utilize molds with multiple plates. Traditionally, the movement of these plates has been controlled by strain linkage mechanisms attached to the plates. As the mold opens, the moving plates are picked up one at a time via the linkages. Springs positioned between the plates are also used to sequence the opening of the plates upon the opening of the mold. The springs are compressed when the mold is closed and expand when the mold is opened to effect lateral movement of the plates in the opening sequence.
While the springs and linkage mechanisms effectively sequence the movement of the plates, they do not restrict the movement of the plates and allow for some freedom of movement. This freedom of movement causes the plates to essentially "float" during operation as the mold is opened and closed. If the molding system is run at high clamp speeds (i.e., cycle), the acceleration of the floating plates increases to the point where the plates strike one another with considerable force, damaging the finely machined surface of the mold plates as well as the heads of the shoulder bolts which serve as the linkages connecting the plates. Due to machining costs, the molds themselves are highly expensive and typically cost in the range of $40,000 to $120,000 each. Thus, any damage incurred during the molding operation can easily result in several more thousands of dollars in labor costs to repair or replace the mold.
One of the reasons for the high cost of the molds is the size of the molds themselves, which must be designed to fit the springs and bolts into the given area. Most designs sacrifice the size of the mold, using larger molds to accommodate the size of the plate control mechanism. This, in turn, leads to the use of larger molding machines, producing a subsequent increase in the price of the finished product.
Even when the molds are run at low clamp speeds, the repeated impact between plates damages the bolt heads and system failure occurs often. The low clamp speeds at which the system must be run correspondingly limit the amount of parts which are produced. Consequently, the operating cost of the system is increased and must be passed on to the customer. Typically, the machine costs are approximately 60 percent of the total cost of the finished product in molds utilizing these types of plate control mechanisms.
To protect the mold, most injection molding machines have a low pressure safety system which prevents the closing of the mold when foreign materials are in the mold. Heretofore, this low pressure safety system could only be used in connection with two plate molds, in which the sequencing of the separation of the plates is unnecessary. In molds with springs used to sequence the moving plates, the force of the springs must be overcome to force the plates together and close the mold. In some molds, this can amount to as much as 2 tons of pressure. With the large amounts of pressure required to overcome the spring forces, the low pressure safety system is no longer effective and foreign material is often crushed between the mold plates, causing damage to the finely machined surfaces of the mold.
Thus, a need exists for a mold plate control mechanism which allows for the positive control and sequencing of moving plates in an injection molding system at high clamping rates. Furthermore, the need exists for a low cost mechanism of this type which can be used with a low pressure safety system.